Capacitors are the basic energy storage devices in memory cells of memory devices such as random access memories (e.g., dynamic random access memory (DRAM) devices, static random access memory (SRAM) devices, and ferroelectric random access memory (FeRAM) devices. Generally, such capacitors consist of two conductive elements (e.g., metal or polysilicon plates), which act as the electrodes of the capacitor (i.e., the storage node electrode and the cell plate capacitor electrode). In the capacitors, the electrodes are insulated from each other by a dielectric material.
The ability to densely pack storage memory cells while maintaining required capacitance levels is a crucial requirement of semiconductor manufacturing technologies if future generations of expanded memory array devices are to be successfully manufactured. As memory devices increase in memory cell density, it is necessary to decrease the size of circuit components, such as capacitors. Thus, there is a continued challenge to maintain sufficiently high storage capacitance while decreasing cell area. It is desirable that each capacitor possesses as much capacitance as possible. Preferably, such capacitors should possess at least about 20×10−15 farads/cell, and more preferably at least about 60×10−15 farads/cell, of charge storage capacity.
The capacitance of a capacitor is dependent upon the dielectric constant of the dielectric material between the electrodes of the capacitor, the distance between the electrodes, and the effective area of the electrodes. One way to retain (or even increase) the storage capacity and decrease its size is to increase the dielectric constant of the dielectric layer of the storage cell capacitor. The dielectric constant is a value which is characteristic of a material. Generally, the dielectric constant is proportional to the amount of charge that can be stored in a material when it is interposed between two electrodes. Further, generally, the dielectric constant is the ratio of the capacitance having a given dielectric material to that of the same capacitor having only a vacuum as the dielectric material.
Various high dielectric constant materials have been utilized in capacitors. For example, metal oxides such as TiO2, WO2, Ta2O5, BST, and Al2O3 have been used. Further, other relatively high dielectric constant materials include silicon nitride (Si3N4) and silicon dioxide/silicon nitride composite layers. As used herein, high dielectric constant materials include any materials used for capacitor dielectrics having dielectric constants of about 10 or more.
However, a major problem arises in the implementation of high dielectric constant materials for use in capacitors, such as DRAM capacitors. Generally, for example, after capacitors have been formed using conventional processes for the fabrication of a wafer including many memory devices, various other layers are formed relative to the capacitors to complete the wafer being fabricated. For example, interconnect layers are deposited, insulative materials are deposited, coatings are applied, etc. Many of such layers require the use of high temperatures (e.g., about 400° C. to about 1000° C.) relative to ambient temperature. For example, such post capacitor formation processes may include an annealing or a densification step at a relatively high temperature for a particular material which has been deposited, or further may include the alloying in a hydrogen atmosphere of a complete or almost complete wafer near the end or at the end of wafer processing. As a result of such thermal cycling, e.g., alloying in hydrogen at a relatively high temperature, high dielectric constant materials exhibit loss of oxygen or reduction of such materials during such post capacitor formation thermal cycling. Such a loss of oxygen results in electrically leaky films.